This invention relates to pressure tight pipe connections and more particularly to a stepped, straight, buttress threaded connection, with internal and external tapered metal-to-metal seals, and perpendicular, positive, stop shoulders, and cylindrical alignment lands adjacent the seals, such connections being known to the trade as HYDRIL tubular connections. Such connections are described in some detail in the Hydril catalogue, 1977 edition, the disclosure of which is incorporated herein by reference and a copy hereunto annexed.
Such connections are used for connecting lengths of production tubing used in wells, e.g. oil and gas wells. The connections may be made up once and not thereafter broken. This is a different use from connections used for drill pipe and work-over drill strings where the connections are repeatedly made up and broken out as the pipe is run in and out of the hole.
For drill pipe, where there is sufficient room for thick walled connections, rugged tool joint type connections are employed. Tool joint connections employ heavy preload to insure sealing of the large area metal-to-metal seals. The presence of drilling fluid inside and outside the drill pipe limits the pressure differential to be withstood by the connections to approximately the pump pressure.
In the case of work-over drill strings, the sealing problem is more difficult than with drill pipe. Not only are the connections repeatedly made and broken but the work-over string is run inside existing production tubing without killing the well and hence without the benefit of drilling fluid to balance the formation pressure. The fluid inside the production tubing outside the work-over pipe may be at very high pressure, e.g. 10,000 psi or more. In addition, the fluid being produced by the well may contain salt water or other corrosive material.
Furthermore, the consequences of a leak in a work-over string are much more serious than with drill pipe. If there is a leak in a work-over drill string, it may be necessary to kill the well in order to make possible the safe removal of the drill string. Killing a well by pumping in mud or other heavy material to shut off and balance the formation pressure may permanently damage the well, and in any event will probably necessitate additional operations such as swabbing and perhaps fracturing to restart a proper flow. In the case of drill pipe, on the other hand, the well has not yet been brought in, it is already under the control of drilling mud.
For work-over drill strings, it is usual to use type "CS" (trademark) Hydrill connections, as shown on page 10 of the Hydril 1977 catalogue. Such connections have a bore shoulder providing the final positive stop. It is suggested in the Hydril catalogue, at page 10, that
"The reserve torque strength in Hydril "CS" tubing connections has actually permitted operators to use their strong "NS-80" Hydril "CS" tubing strings as drill out strings--making round trips as needed, drilling and working as needed and then setting the same string as production tubing." PA1 2,259,232--Stone PA1 2,992,019--MacArthur. PA1 2,102,072--Hinderliter PA1 2,107,716--Singleton PA1 2,110,825--Archer PA1 2,907,589--Knox PA1 3,100,656--MacArthur PA1 3,336,054--Blount et al PA1 3,667,784--Hokanson et al PA1 2,110,127--Hinderliter PA1 2,150,221--Hinderliter (FIG. 4) PA1 "In FIGS. 3 and 4 is illustrated a modification in which the male member 115 which may correspond to either the upset end of a pipe or the threaded shank of a pin member is screwed into a tapered socket 119 in a tool joint member 110. The lower end of the tapered socket 113 is not threaded but instead is provided with a tapered or inclined smooth wall 116 adapted to wedgingly cooperate with a rubber sealing ring 117 disposed in an annular groove 118 in the lower end of the male member 115." PA1 "By reason of the fact that the socket 113 is tapered, the rubber ring 117 is enabled to clear the threads until it reaches the bottom of the socket. When the ring comes into engagement with the inclined wall 116, the inserting thrust of the male member will cause it to be wedged into tight sealing engagement with the inclined or tapered wall 116 as shown in FIG. 4." PA1 "The modification of the invention illustrated in FIGS. 7 and 8 is similar to that shown in FIGS. 3 and 4 with the exception that in this form of the invention the outer periphery of the rubber sealing ring 217 is inclined or tapered and the wall 216 is substantially straight. PA1 The rubber sealing ring 217 is seated in a groove 218 in the lower end of the tapered threaded male member 215 screwed into the tapered threaded socket 213 of the tool joint member 210. PA1 The outer periphery of this rubber ring 217 is inclined upwardly and outwardly from the lower end of the shank 215 as indicated at 220. The bottom of the socket 213 terminates in a substantially straight wall 216 adapted to wedgingly cooperate with the sealing ring 217. PA1 In FIG. 7 I have illustrated the condition of the rubber sealing ring 217 prior to its being wedged or crowded down the wall 216, whereas in FIG. 8 I have illustrated its position after it is wedged into contact with the wall 216 by the inserting thrust of the male member 215." PA1 4,009,893--Schatton et al (FIG. 2) PA1 4,085,951--Morris PA1 "In oil fields, drill strings are frequently connected by Hydril-type connectors, which involve a two step thread design, as is well known to those in the art. For instance, Hydril-type connectors are disclosed in U.S. Pat. No. 2,532,632, the disclosure of which is hereby incorporated by reference for the teachings of the Hydril-type connectors therein. PA1 The prior art has made extensive use of "O" rings and similar sealing means, in the sealing of pipes and the like. See, for instance, U.S. Pat. Nos. 3,054,628; 3,167,333; 2,980,451; 2,889,733; 2,813,567; 2,770,477; and 2,553,340. Generally the sealing rings or gaskets of these prior art patents have been made of Teflon or similar plastic material. PA1 The use of rings of plastic or rubber, or the like, in rotary tools is also known to the art, see, for instance, U.S. Pat. Nos. 2,102,072 and 2,110,127. Finally, the use of Teflon as a sealing ring in Hydril-type connectors is disclosed in U.S. Pat. Nos. 2,907,589 and 3,100,656. In these patents, the Teflon seal ring is located in the area of the free terminal forward end of the pin member of the Hydril-type connector. If such sealing rings are located on the pin member, they are highly susceptible to damage, and if located on the box member they are quite difficult to install, and subject to a tendency to be locally deranged in an unpredictable manner as the joint is made up. PA1 As acknowledged hereinabove, many different designs of screw threaded pin and box type tubing and drill pipe joints have been proposed by the prior art. However, in spite of the many proposals, a number of problems have remained unsolved, especially those problems having to do with a method of adequately and enduringly sealing a Hydril-type connector after the metal-to-metal seating surfaces have been damaged, which is a frequent occurence, against the extremely high fluid pressures which are encountered in deep drilled oil and gas wells. For instance, such pressures might range as high as 15,000 psi. PA1 The rubber O-rings and compressible rubber gaskets, such as those acknowledged hereinabove, have from time to time been used to seal pipe and tubing joints, and under certain operating conditions, especially when relatively low pressures are involved, they have proven to be thoroughly satisfactory. However, such seals have not been suitable for use under the high temperature and pressure conditions which are encountered in deep oil and gas wells. For one thing, these relatively thin rubber rings tend to be badly damaged, and often rendered useless, by being extruded under the well pressure into small clearances between the joint members at the seal ring groove."
As above suggested, a work-over operation is usually a drill out procedure. Something is wrong with an existing well; it is not flowing at the expected rate. A slender work-over string, e.g. one inch inner diameter or one and five-sixteenths inch outer diameter, is snubbed in through the production tubing, and the bottom of the well is drilled out to remove, for example, an accumulation of shale. Or the bottom of the well may be washed out.
The work-over string is run in through a small blow out preventer which closes the annulus between drill string and production tubing. A suitable check valve at the bottom of the string prevents flow up the work-over string. If a leak develops in the work-over string, it is difficult to remove the work-over string without killing the well, since the work-over string fills up with high pressure fluid and it is not safe to break the connections in the string as required for its removal.
The metal-to-metal seals of Hydril connections may not be expected to maintain their pressure tightness after repeated make up and break out, and in many cases today are not even initially rated as high as the pressures that may encountered in the course of working over an existing well. To insure against leakage it is customary to supplement the metal-to-metal seals with an O ring seal. Such seals have been placed in the box. However, the installation of the seals and the cleaning of the grooves on such small diameter parts is difficult.